Field of the Invention
The invention relates to a packaging material for packaging a flat electrochemical cell main body containing an electrolyte (for example, liquid and solid electrolytes), as typified by batteries or capacitors. In more detail, the invention relates to an adhesive sheet for sealing metal terminals of a flat electrochemical cell which is interposed between a packaging body for packaging a flat electrochemical cell main body and metal terminals connected to each of a positive electrode and a negative electrode of the flat electrochemical cell main body, thereby adhering the packaging body to the metal terminals and also adhering the packaging bodies to each other with unconnected ends of the metal terminals protruding outside.
Description of the Related Art
As one of flat electrochemical cells, a lithium battery is exemplified. The lithium battery is also called a lithium secondary battery and is a battery containing, as an electrolyte, a solid high polymer material, a high polymer material in a gel form, a liquid and the like and generating electricity due to movement of a lithium ion. The lithium battery includes ones in which positive electrode/negative electrode active substances are each composed of a high polymer. The lithium secondary battery is configured of a lithium battery main body composed of positive electrode collector material (for example, aluminum)/positive electrode active substance layer (for example, metal oxides, carbon black, metal sulfides, electrolytic solutions, and high polymer positive electrode materials such as polyacrylonitrile)/electrolyte (for example, carbonate based electrolytic solutions such as propylene carbonate, ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate, inorganic solid electrolytes composed of a lithium salt, and gel electrolytes)/negative electrode active substance layer (for example, lithium metal, alloys, carbon, electrolytic solutions, and high polymer negative electrode materials such as polyacrylonitrile)/negative electrode collector material (for example, copper), a packaging body for packaging them, and the like. With respect to applications of the lithium secondary battery, the lithium secondary battery is used in personal computers, portable terminals (for example, mobile phones and PDA), video cameras, electric cars, storage batteries for energy storage, robots, satellites, and the like. In this specification, the positive electrode collector material and the positive electrode active substance layer are referred to as a positive electrode; and the negative electrode collector material and the negative electrode active substance layer are referred to as a negative electrode, respectively.
As the packaging body of a lithium battery, a metallic can obtained by press working a metal into a container in a cylindrical or rectangular parallelepiped form, a bag body obtained by working a laminate obtained by laminating a plastic film and a metal foil of aluminum or the like into a bag-like form or the like (this bag body will be hereinafter referred to as “packaging body”), and the like are useful.
Now, a packaging body composed of a metallic can is rigid, and the shape of a battery itself is determined by this.
Accordingly, for example, when this is used for a mobile phone, there is involved a problem that since the size of a mobile phone main body is determined by the shape of the battery, the shape of the mobile phone cannot be freely designed. Then, there is a tendency that the foregoing packaging body is used because it is flexible so that the shape of the mobile phone main body can be freely designed to some extent. Furthermore, the reasons why there is a tendency that the foregoing packaging body is used are as follows. That is, when a battery is used at a high temperature and an internal pressure abnormally increases, there is a problem that a packaging body composed of a metallic can is dangerous because it withstands until explosion or ignition occurs. On the other hand, in the foregoing packaging body which is sealed in a thermal-adhesive part, the thermal-adhesive part is separated, whereby the packaging body works as a safety valve for releasing an internal pressure. Accordingly, though this packaging body loses the function as a battery, it is possible to reduce danger of explosion or ignition as compared with a packaging body composed of a metallic can.
From the standpoints of physical properties necessary for the lithium battery, workability, economy and the like, as illustrated in FIG. 2, a laminate A obtained by laminating at least a substrate layer A1, a barrier layer A2 composed of a metal foil of aluminum or the like and a thermal-adhesive resin layer A3 is used as the foregoing packaging body. This laminate A is worked into a bag body as illustrated in FIG. 3A (though the bag body in FIG. 3A is a pillow-type packaging bag, it may be a packaging bag of a three-side type or a four-side type or the like), thereby containing therein the lithium battery main body and metal terminals 31 connected to a positive electrode and a negative electrode of the lithium battery main body, respectively and an opening is sealed by thermal-adhesion with unconnected ends of the metal terminals protruding outside. Alternatively, as illustrated in FIG. 4A, this laminate A is press molded to form a recess in such a manner that the thermal-adhesive resin layer is located inward; the lithium battery main body and the metal terminals 31 connected to a positive electrode and a negative electrode of the lithium battery main body, respectively are contained in this recess with unconnected ends of the metal terminals protruding outside; the recess is covered such that the thermal-adhesive resin layer of a separately prepared laminate A in a sheet form (not illustrated) is located on the side of the recess; and the edge is sealed by thermal-adhesion. The thus formed material is used as a lithium battery 10 as illustrated in FIG. 3B or FIG. 4B. A symbol S denotes a thermal-adhesive part.
The thermal-adhesive resin layer A3 configuring the packaging body (laminate A) is required to have thermal-adhesive properties between the thermal-adhesive resin layers A3 each other and thermal-adhesive properties to the metal terminals 31 protruding outside of the packaging body from the lithium battery main body. For such a thermal-adhesive resin layer, acid-modified polyolefin based resins having excellent adhesive properties to a metal, for example, polyolefin resins graft-modified with an unsaturated carboxylic acid, copolymers of ethylene or propylene with acrylic acid or methacrylic acid, metal-crosslinked polyolefin resins, and the like were used.
However, when the acid-modified polyolefin based resin is used in the thermal-adhesive resin layer A3 configuring the packaging body (laminate A), there are a problem that in comparison with a general polyolefin based resin (this resin mean a linear or branched olefin based resin composed of carbon and hydrogen; hereinafter referred to as “general polyolefin based resin”), its splipperiness is poor and when worked in a bag-like form, wrinkles are formed; and a problem that there is a possibility that when press molded to form a recess, pinholes or cracks are generated. Then, there has become employed a method in which as a replacement of this, a general polyolefin based resin is used in the thermal-adhesive resin layer A3 and an adhesive sheet for sealing metal terminals of a lithium battery composed of a single layer of the foregoing acid-modified polyolefin based resin which can be thermally adhered to both of this general polyolefin based resin and the metal terminals 31 or plural layers thereof formed by putting the acid-modified polyolefin based resin on at least one surface layer of the single layer (this adhesive sheet for sealing metal terminals of a lithium battery will be sometimes referred to as “adhesive sheet”) is made to intervene between the metal terminals and the thermal-adhesive resin layer, followed by thermal-adhesion to achieve sealing.
Concretely explaining, as illustrated in FIG. 5, the respective metal terminals 31 (see FIG. 3B and FIG. 4B) are protruded to the outside of the packaging body from a lithium battery main body 30 prior to injecting an electrolyte, and for example, an adhesive sheet 1′ for sealing metal terminals composed of the foregoing acid-modified polyolefin based resin single layer is fixed on both surfaces of the metal terminals 31 with a temporarily adhesive seal. The lithium battery main body 30 is contained in the recess of the laminate A having a recess formed therein by press molding as illustrated in FIG. 4A; the recess is covered by a separately prepared laminate A in a sheet form (not illustrated); three edges including the edge provided with the metal terminals 31 of the lithium battery main body 30 are thermally adhered; and an electrolyte is then injected from the edge of one non-adhesive part; and thereafter, this edge is sealed by thermal-adhesion. A lithium battery 10 as illustrated in FIG. 4B is thus formed.
Now, the respective metal terminals 31 of the lithium battery 10 is thermally adhered in a site having the adhesive sheet 1′ provided therein in a state that it is interposed by the packaging body (laminate A). At that time, the metal terminals 31 has a thickness of at least about 50 μm and a width of at least about 2.5 mm. In order to ensure a sealed state by filling spaces of both side parts of the metal terminals 31 with the adhesive sheet 1′ and the thermal-adhesive resin layer A3 of the packaging body (laminate A), heat and pressure for thermal-adhesion are necessary. Accordingly, there was a problem that the adhesive sheet 1′ and the thermal-adhesive resin layer A3 of the packaging body (laminate A) are pushed out to the outside of a pressurized part, whereby the pressurized part becomes thin-walled. Also, in general, in cutting in a small width, burrs of from several μm to several tens μm are formed in the both side ends of the metal terminals 31. As a result, there was a problem that the barrier layer A2 of the packaging body (laminate A) which is composed of a metal foil of aluminum or the like comes into contact with the metal terminals 31 to cause a short circuit.
Also, in order to improve this problem, as the adhesive sheet 1′ to be used for the both surfaces of the metal terminals 31, a battery terminal covering material composed of a laminated film having a three-layer configuration in which both surfaces of a heat resistant resin film are covered with a film which is heat sealable to an inner film of a battery packaging material (this battery terminal covering material is corresponding to the foregoing adhesive sheet for sealing a metal terminals) is proposed (see, for example, Patent Document 1). However, when the battery terminal covering material as described in Patent Document 1 is used, though a phenomenon of the generation of the foregoing short circuit is improved as compared with one composed of an acid-modified polyolefin based resin single layer, a layer-to-layer adhesive strength between the heat resistant resin film and the film which is heat sealable to the inner film of the battery packaging material is not sufficient. For that reason, when battery components causes abnormal heat generation, a gas is generated in the inside of the battery and an internal pressure of the battery increases, there was a possibility that separation takes place at an interface between the heat resistant resin film and the film which is heat sealable to the inner film of the battery packaging material due to a stress for separating the battery packaging material and the metal terminals from each other in the parts where the metal terminals are held. In addition to the lithium battery containing a lithium battery main body therein, when a capacitor or an electric double layer capacitor is contained, the same problem is also caused.